Free-radical polymerization is the most widely applicable technique for polymerizing vinylic monomers. It permits the polymerization of a multiplicity of monomers varying in structure, functional groups and polarity. The copolymerization of different monomers with one another is also possible. Owing to unavoidable side reactions such as chain transfer, disproportionation, recombination or elimination, however, it is very difficult to control the molecular weight distribution. Normally, polymers having a polydispersity index PDI of 2.0 or more are obtained. PDI is defined as ##EQU1## where M.sub.w is the weight-average and Mn the number-average molecular weight.
A process which has long been known for preparing polymers with a narrow molecular weight distribution is that of anionic polymerization. However, it is only useful with a limited number of monomers. Apolar monomers, such as styrene or butadiene, can be polymerized anionically. In the case of polar monomers, such as n-alkyl acrylates, for example, anionic polymerization is very difficult. Moreover, anionic polymerization requires highly pure monomers and solvents, and the complete exclusion of atmospheric humidity.
Another method of preparing polymers of narrow molecular weight distribution is that of controlled free-radical polymerization, sometimes also called "living" free-radical polymerization, which is described, for example, in M. K. Georges et al., Trends in Polymer Science, Vol. 2, No. 2 (1994), pages 66 to 72. The fundamental strategy of this method consists in temporarily blocking and then reactivating, in a controlled manner, the reactive free-radical chain ends of a growing polymer chain. The dynamic equilibrium between active and dormant form leads to a low steady-state concentration of free polymer radicals.
A variety of techniques are available for blocking and stabilizing the free-radical chain end. They employ stable free radicals and/or metal salts.
For instance, it is known to use "iniferters", i.e. free-radical generators which both free-radically initiate a polymerization and terminate the chain end by combination. Examples of photochemically activated iniferters, such as dithiocarbamates, are described in T. Otsu et al., Eur. Polym. J., Vol. 25, No. 7/8 (1989), pages 643 to 650. These photochemical iniferters, however, are very expensive compounds, and photochemically initiated polymerization is highly uneconomic in industrial practice. Furthermore, the polydispersity index is very high in some cases. There are also thermal iniferters, such as tetramethylene disulfides, which are described, for example, in K. Endo et al., Macromolecules, Vol. 25 (1992), pages 5554 to 5556. In this case the PDI, at levels of between 3 and 4, is too high to be satisfactory.
EP-A 135 280 describes the use of stable N-oxyl radicals, which combine reversibly with the reactive chain ends. However, this process produces not high molecular mass-polymers but only oligomers instead.
U.S. Pat. No. 5,322,912 discloses cyclic, sterically shielded N-oxyl radicals which are used in combination with conventional initiators. These systems, however, do not permit the polymerization of alkyl acrylates.
EP-A 489 370 describes free-radically initiated addition polymerization in the presence of alkyl iodides. Here too, the molecular weights are at an unsatisfactorily low level.
The same disadvantage is shown by the products described in EP-A 222 619, which are prepared with the aid of bimetallic catalysts containing cyclopentadienyl ligands.
All methods known to date have the disadvantage that the additives used to control the reaction are very expensive and the processes are therefore uneconomic.